US Cellular VS In Store Purchase

Phones sold directly by Apple are unlocked so can be used with pretty much any carrier.

Phone's sold by U.S. cellular will likely be locked to them, and would require a period of usage with them before they can be unlocked and then used with anyone else.

As to why there is no U.S Cellular option on the Online Apple Store, that's down to agreements made between the carriers and Apple. U.S. Cellular may not have entered into one with Apple.

There is an exception to Apple cellphones being unlocked:

" Will my new iPhone be unlocked?

Nearly all iPhone models sold on apple.com and at the Apple Store are unlocked. This means they aren’t tied to a single carrier. The exception is when you buy an iPhone with an AT&T Installment Plan. It will be tied — or locked — to AT&T.

Your iPhone will be unlocked when you:

• Choose any carrier and select One-time payment.
• Choose any carrier and select Apple iPhone Payments.
• Purchase a SIM-free model. This is like any other iPhone, just without a carrier nano-SIM card.

Your iPhone will be locked when you:

• Choose AT&T as your carrier and pay with an AT&T Installment Plan.

You'd have to ask US cellular why they haven't negotiated that with Apple.